In an LED illumination field, a silicon-controlled-rectifier (SCR) dimming circuit is widely applied and the SCR dimming circuit features simple circuit structure, low cost, convenient use and strong controllability. With increase of market share of LED illuminating products, the SCR dimming circuit also plays an increasingly important role.
FIG. 1 is schematic diagram illustrating a structure of a circuit of an existing SCR module (SCRA). The SCR module has two states of turning on and turning off. After an SCR S1 is turned on, a holding current may flow through the SCR module (SCRA) to maintain the thyristor S1 in the turning-on state. Ideally, no current flows through the SCR module when the SCR is turned off. However, as shown in FIG. 1, the SCR is a modular structure and the module device cannot be in an ideal state. At the moment of turning off the SCR S1, there is still a current flowing through the SCR module. The current is a leakage current of the SCR module. Usually, the leakage current of the SCR module is very small with its value approximately being several milliamperes to a dozen of milliamperes and the holding current is greater than the leakage current.
An existing LED SCR dimming circuit performs dimming by using the above SCR module. As shown in FIG. 2, the LED SCR dimming circuit includes an SCR module 10, a rectifier module 20, and a constant current module 30. When a voltage on both ends of an LED lamp string is lower than a set voltage, the LED string is not turned on and there is no current flowing through. To reliably turn on the SCR module 10 at a particular angle, a bleeder circuit may be set in the LED dimming circuit. The bleeder circuit includes a power tube M1 and a resistor R3 for providing a current to the SCR module at the time of no current in the LED lamp string. Generally, a value of a bleeder current set by the bleeder circuit is greater than the holding current of the SCR module to ensure the SCR module 10 operates reliably.
After the SCR is changed from the turning-off state to the turning-on state, the SCR module is in a low resistance state. When a voltage after a phase cut is greater than a set voltage required to turn on the LED, there is a current flowing through the LED lamp string and the conduction current flowing through the SCR module is a current of the LED lamp string; when the voltage after the phase cut is smaller than the set voltage required to turn on the LED, the LED lamp string is not turned on and there is also no current flowing through. The conduction current flowing through the SCR module is mainly limited by the SCR module itself. In this case, the bleeder current is a conduction current flowing through the SCR module and therefore the current does not flow through the LED lamp string and is a loss current. Generally, the conduction current of the SCR is M times its leakage current. Different SCR modules may have different leakage currents. When an LED dimming circuit is designed, a bleeder current of a bleeder circuit is designed to be greater than a maximum leakage current to ensure the LED dimming circuit can be matched with most SCR modules, thereby realizing SCR dimming. Thus, the loss current designed for the bleeder circuit is also large. Particularly, when one SCR module controls N LED modules, because each of the LED modules is provided with a dimming circuit, the total bleeder current flowing through the SCR module may be N times of the bleeder current of a single dimming circuit, resulting in larger energy consumption.
At present, how to control the above bleeder current (in a case that the SCR is turned on and a circuit voltage is smaller than the conduction voltage of the LED) to reduce power source loss caused by the bleeder current and improve power source efficiency becomes a problem to be solved.